1. Field of the Invention
The present invention relates to the art of controlling processes of semiconductor manufacturing apparatuses.
2. Description of the Related Art
In a dry etching process included in the steps for manufacturing semiconductor devices, high-frequency power is applied to an electrode on which the substrate to be processed in the etching apparatus is placed. The high-frequency power is applied via a capacitor, and since more electrons than ions are injected to the electrode due to the differences in the speed of electrons and ions in the plasma, the potential of the electrode becomes negative. The difference between the potential of the electrode and the ground level is called a self-bias voltage. The self-bias voltage enables the ion components in the plasma to be accelerated and injected to the substrate, by which the etching process is promoted.
In other words, though the occurrence of self-bias voltage is inevitable for the etching process, too much voltage may damage the substrate, so it is necessary to set an appropriate self-bias voltage for the etching process.
As described, in the dry etching process included in the steps for manufacturing semiconductor devices, it is necessary to set the self-bias voltage to an appropriate value, but in order to do so, it is necessary to measure the self-bias voltage first. On the other hand, recent etching apparatuses adopt electrostatic chuck mechanisms for holding and cooling the substrate on the electrode.
The electrode equipped with an the electrostatic chuck mechanism has a thin insulating film formed on the surface of the electrode, and DC voltage is applied via the insulating film so as to polarize the insulating film and support the substrate (sample) via the coulomb force by the charge generated between the substrate and the electrode. If the electrode is not equipped with such electrostatic chuck mechanism, the self-bias voltage of the processing substrate can be obtained by measuring the potential of the electrode itself. However, if the electrode is equipped with the above-mentioned electrostatic chuck mechanism, the self-bias voltage cannot be acquired directly from the potential of the electrode, since the resistance of the insulating film is too large.
Therefore, in an etching apparatus utilizing an electrode having an electrostatic chuck mechanism, an additional operation was necessary to measure the self-bias voltage in advance using an electrode having no insulating film just for measuring the self-bias voltage, and then to estimate the self-bias voltage under similar etching conditions.
Japanese Patent Number 3635463 (patent document 1) discloses a plasma processing apparatus capable of measuring a self bias voltage of the object to be processed accurately in a short time and hold the object stably with the required force of electrostatic chuck, providing a circular electrostatic chuck sheet 30 crowned on the upper surface of a mounting stage provided at the center of a processing chamber, with a semiconductor wafer mounted on the electrostatic chuck sheet. The electrostatic chuck sheet is constituted by sealing a thin conducting film formed for example of a copper coil as an electrode for electrostatic chuck between a thin film formed for example of SiC and an insulating film formed for example of polyimide. The thin conducting film is connected to the output terminal of a variable DC power supply through an ammeter, and the ammeter detects the leaking current between the semiconductor wafer and the conducting film, outputting the variable DC voltage under the control of a control unit and the variable DC power supply.
According to the prior art plasma processing apparatus, however, the self-bias voltage Vsb is measured from the leaking current flowing through the electrostatic chuck sheet, which is theoretically possible, but the accurate self-bias voltage can only be acquired if the quality of the insulating film for electrostatic chuck is stable and uniform as a premise. Actually, the absolute value of the leaking current is very small, and the film thickness and quality of the electrostatic chuck sheet are not uniform, so that the SN ratio of the measurement is too small. Therefore, this method is not practical. Even further, since the atmosphere within the processing chamber varies by time and the status of the chuck fluctuates, it is difficult to measure the leaking current in a stable manner. Moreover, if the insulating film is composed of ceramic that does not have a leaking current, it is basically not possible to measure the self-bias voltage according to this method.